Positive electrode material for lithium secondary battery and process for producing the same

ABSTRACT

There is obtained a material of a positive electrode for a secondary lithium-ion cell having high cycle durability and high safety in high-voltage and high-capacity applications, which is a particulate positive electrode active material for a secondary lithium-ion cell represented by a general formula, Li a CO b A c B d O e F f  (A is Al or Mg, B is a group-IV transition element, 0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03, 0.0001≦d≦0.03, 1.98≦e≦2.02, 0≦f≦0.02, and 0.0001≦c+d≦0.03), where element A, element B and fluorine are evenly present in the vicinity of the particle surfaces.

RELATED APPLICATIONS

The present application is based on International Application No.PCT/JP2003/007223, filed Jun. 6, 2003, and claims priority from, JapanApplication Number 2002-279198, filed Sep. 25, 2002, the disclosure ofwhich is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to positive electrode material for asecondary lithium cell that has a large capacity and excellent cyclecharacteristics, particularly under high voltages, and a method formanufacturing the same.

BACKGROUND ART

In recent years, with increase in the production of portable andcordless equipment, demands for small and light non-aqueous electrolytesecondary cells having a high energy density have increased, anddevelopment for non-aqueous electrolyte secondary cells having excellentcharacteristics have been desired much more than before.

As a positive electrode material for non-aqueous electrolyte secondarycells, LiCoO₂, LiNiO₂, LiMn₂O₄ and the like are used, and particularly,a large quantity of LiCoO₂ is used from the aspect of safety, capacityand the like. In this material, since lithium in the crystal latticeescapes into the electrolyte as lithium ions when charged, and thelithium ions are inserted into the crystal lattice from the electrolytewhen discharged, the material manifests the function as the positiveelectrode active material.

Theoretically, one lithium atom can be released from or inserted intoone LiCoO₂ lattice. However, if the majority of lithium are released orinserted, LiCoO₂ is violently deteriorated, and especially cycleproperties are significantly damaged. Therefore, in the present state,only about 0.55 lithium is released from or inserted into one LiCoO₂,and a capacity of only about 150 mAh is used for 1 g of LiCoO₂.

Although increase in the capacity is expected by releasing and insertinga larger quantity of lithium atoms, if lithium are released or insertedin present quantities or more, the violent deterioration of LiCoO₂occurs due to the phase transition of the LiCoO₂ crystal lattice,accompanying damage of particles and the crystal lattice, and theelution of cobalt ions from the crystal lattice, causing a problem ofdifficulty to secure satisfactory cycle properties.

Although there are approaches to improve the cycle durability at 4.5 Vby doping 5% by weight of zirconium into LiCoO₂, the initial capacitylowers significantly, and cycle durability is also unsatisfactory (referto Z. Chen and J. R. Dahn, 11th International Meeting of LithiumBattery, Jun. 23-28, 2002, Monterey, USA, Abstract No. 266).

Therefore, the object of the present invention is to provide a positiveelectrode active material for a high-capacity and highly safe lithiumion secondary cell for high voltage that excels in prevention ofdeterioration at high voltage, and excels in cycle durability.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, the present inventors diligentlyconducted repetitive studies, and found that a secondary lithium cellhas good cycle characteristics even in a high-voltage regionconventionally said to be overcharging, by simultaneously adding aspecific quantity of a plurality of specific metal elements to a lithiumcobaltate-based particulate positive electrode active material forsecondary lithium cells, or further simultaneously adding fluorine.

In the present invention, high voltage means a voltage wherein chargevoltage is 4.4 V or higher on the basis of a lithium electrode.Furthermore, as a specific charge voltage, 4.5 V is exemplified. At thistime, a capacity of about 185 to 190 mAh can be used for 1 g of LiCoO₂,which corresponds to the deintercalation of about 0.7 lithium atom forone LiCoO₂.

In the present invention, although the reason why good cycle propertiescan be obtained in the high-voltage region is not quite clear, it isconsidered that under a high-voltage condition wherein a majority oflithium ions are extracted, the specific metal elements, which aresimultaneously added, and present on the surface of the particles orpartially dissolved on the surfaces of particles, may act as the pillarsof the crystal lattice to reduce the strain of the crystal latticecaused by phase transition or expansion/contraction, thereby suppressingits deterioration. At the same time, since the chance wherein cobaltatoms directly contact the electrolyte is reduced, and no overchargedstates occur locally in the particles, it is considered thatdeterioration is suppressed.

Thus, the positive electrode material for secondary lithium-ion cells ofthe present invention is a material of a positive electrode for asecondary lithium cell characterized in being a particulate positiveelectrode active material for a secondary lithium cell represented by ageneral formula, Li_(a)Co_(b)A_(c)B_(d)O_(e)F_(f) (A is Al or Mg, B is agroup-IV transition element, 0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03,0.0001≦d≦0.03, 1.98≦e≦2.02, 0≦f≦0.02, and 0.0001≦c+d≦0.03); and thatelement A, element B and fluorine are evenly present in the vicinity ofthe particle surfaces.

In the present invention, “evenly present” means not only the casewherein each of the above-described elements is substantially evenlypresent in the vicinity of the particle surfaces, but also the casewhere the quantity of each of the above-described elements presentsbetween particles of substantially the same; and it is sufficient ifeither one is satisfied, especially, it is preferable that the both aresatisfied. Specifically, it is particularly preferable that the quantityof each of the above-described elements is substantially the same, andeach of the above-described elements is evenly present on the surface ofa particle.

In the present invention, it is preferable that at least a part of theelement represented by A or B contained in the particulate positiveelectrode active material for a secondary lithium-ion cell substitutesfor cobalt atoms in the particles to form a solid solution. Thepreferable atomic ratio of the element A to the element B is0.33≦c/d≦3.00 and 0.002≦c+d≦0.02.

In the present invention, although the element A is either aluminum ormagnesium, and the element B is selected from group-IV transitionelements, it is preferable that the element A is magnesium. When theelement A is magnesium, it is considered that magnesium substitutesmainly the lithium site. It is also preferable that the element B iszirconium.

The present invention also provides a material of a positive electrodefor a secondary lithium cell characterized in that no diffraction peaksare observed at 2 θ of 28±1° in a high-sensitivity X-ray diffractionspectrum using Cu—K α ray.

In the present invention, the high-sensitivity X-ray diffractionspectrum means a diffraction spectrum obtained when the acceleratingvoltage of the X-ray tube is 50 kV, and the accelerating current is 250mA. The ordinary X-ray diffraction spectrum uses an accelerating voltageof 40 kV and an accelerating current of about 40 mA, and this isdifficult to detect a trace of the impurity phase noted in the presentinvention, and significantly affecting the cell performance whilesuppressing analysis noise at a high accuracy in a short time.

For example, when the element B is zirconium, the bonding state ofcobalt atoms, lithium atoms and oxygen atoms can be determined byhigh-sensitivity X-ray diffraction spectrum. When the element B iszirconium, and forms a solid solution with cobalt atoms, no diffractionspectra derived from a single-component oxide of zirconium (ZrO₂) areobserved, and only the spectrum of Li₂ZrO₃ is partly observed. Thespectrum intensity of Li₂ZrO₃ is affected by the feeding mole ratio ofzirconium to the element A, the firing method or the like.

Specifically, if the element B does not form a solid solution withcobalt atoms, diffraction spectra derived from a single-component oxideof the element B are significantly observed. From the diffractionspectrum intensity of the single-component oxide of the element B, thequantity of the element B in the solid solution can be observed. Theelement B occupies the cobalt site substitutionally to form a solidsolution, and the extent of solid solution is preferably 60% or more,and more preferably 75% or more.

The present inventors found that the cell performance was improved ifthe quantity of the element B as the single-component oxide was small.Therefore, the present invention provides a material of a positiveelectrode for a secondary lithium cell, characterized in that theabundance of the single-component oxide of the element B is 20% or less.

The abundance of the single-component oxide of the element B exceeding20% is not preferable, because the effect of improving charge-dischargecycle durability at high voltages lowers. The abundance of thesingle-component oxide of the element B is particularly preferably 10%or less.

Particularly, the present inventors selected zirconium as the element B,selected magnesium as the element A, and found that the material for thepositive electrode having a specific structure obtained by manufacturingusing a specific method by coexisting these has significantly improvedcharge-discharge cycle durability at high voltages.

Here, it is important that in the specific structure, specifically,added zirconium is not present as a single-component oxide on thesurface of lithium cobaltate particles; and for that, the presentinventors found that it was particularly preferable to add magnesium tozirconium in the above-described specific atomic ratio (0.33≦c/d≦3.00and 0.002≦c+d≦0.02), and lithium cobaltate was formed in the coexistenceof a zirconium compound and a magnesium compound. In other words, thepresent inventors found that the coexistence of magnesium had thesignificant effect to raise the reactivity of zirconium. It was alsoknown that the existence of magnesium had the effect to lower the Co₃O₄content in formed lithium cobaltate.

Although the action mechanism wherein unique property improvement can beachieved by the simultaneous addition of zirconium and magnesium has notbeen clarified, it is estimated that an inactive film is evenly formedon the surfaces of lithium cobaltate particles by the simultaneousaddition of zirconium and magnesium, and crystal disintegration of thecrystal structure of lithium cobaltate from particle surfaces caused bycharge and discharge can be suppressed.

In the present invention, although unique property improvement observedby the simultaneous addition of zirconium and magnesium has beendisclosed, the combination of such elements is not limited to thiscombination, but the combination of the element A for raising thereactivity of the element B can be selected so that the added element Bis not present as a single-component oxide. As the element A, especiallymagnesium is preferably adopted as described above.

The present invention is also characterized in that the particulatepositive electrode active material for a secondary lithium cell consistsof particles wherein 10 or more primary particles are coagulated to forma secondary particle, and the average particle diameter of the secondaryparticle is 2 to 20 μm. By such a secondary particle structure ofcoagulated bodies, the improvement of the packed density of the activematerial of the electrode layer, and the improvement of large-currentcharge-discharge properties can be achieved.

As a preferable method for manufacturing a material of a positiveelectrode for a secondary lithium-ion cell characterized in being aparticulate positive electrode active material for a secondary lithiumcell represented by a general formula, Li_(a)Co_(b)A_(c)B_(d)O_(e)F_(f)(A is Al or Mg, B is a group-IV transition element, 0.90≦a≦1.10,0.97≦b≦1.00, 0.0001≦c≦0.03, 0.0001≦d≦0.03, 1.98≦e≦2.02, 0≦f≦0.02, and0.0001≦c+d≦0.03); and that element A, element B and fluorine are evenlypresent in the vicinity of the particle surfaces, the present inventionprovides a method for manufacturing a material of a positive electrodefor a secondary lithium cell that consists of particles wherein 10 ormore primary particles are coagulated to form a secondary particle,characterized in that a cobalt material containing at least eithercobalt oxyhydroxide or cobalt hydroxide, lithium carbonate, and theelement A and the element B are mixed and fired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an X-ray powder diffraction spectrum of thepositive electrode active material obtained in Example 2;

FIG. 2 is a graph showing an X-ray powder diffraction spectrum of thepositive electrode active material obtained in Example 10, and

FIG. 3 is a graph showing an X-ray powder diffraction spectrum of thepositive electrode active material obtained in Comparative Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION

As described above, the particulate positive electrode active materialfor a secondary lithium cell of the present invention has a generalformula, Li_(a)Co_(b)A_(c)B_(d)O_(e)F_(f). In this general formula, itis preferable that a is 0.90 to 1.10, b is 0.97 to 1.00, c is 0.0001 to0.03, d is 0.0001 to 0.03, e is 1.98 to 2.02, f is 0 to 0.02, and c+d is0.0001 to 0.03.

The element A is preferably Al or Mg and the element B is preferably atleast one element that belongs to the group IV of the periodic table. Asthe group-IV of the periodic table, Ti, Zr and Hf are exemplified.

The positive electrode active material for a secondary lithium cell ofthe present invention is preferably spherical and particulate, and theaverage particle diameter thereof is preferably 2 to 20 μm, morepreferably 3 to 15 μm.

If the average particle diameter is smaller than 2 μm, it is difficultto form a dense electrode layer; on the contrary, if the averageparticle diameter exceeds 20 μm, it is difficult to form a flat surfaceof the electrode layer.

The positive electrode active material is preferably a particulatematerial wherein 10 or more primary particles of fine particlescoagulate to form a secondary particle. Thereby the packed density ofthe active material of the electrode layer can be improved, and theimprovement of large-current charge-discharge properties can beachieved.

In the particulate positive electrode active material of the presentinvention, the element A or B, or F must be substantially evenly presenton the surfaces of particles thereof. In other words, the element A orB, or F must not be substantially present in the particles. In such acase, since the element A or B, or F is present on the surface of thepositive electrode active material, the effect can be obtained by addinga small quantity. If the element A or B, or F is present in theparticles, the effect of the present invention cannot be obtained.

When the element A or B, or F is added in the particles in order toobtain the effect of the present invention, that is, the high capacityand high cycle properties, under the use of high voltage as the positiveelectrode active material, the addition of a large quantity of theelement A or B, or F is required.

However, since the addition of a large quantity of the element A or B,or F rather causes the lowering of initial capacitance, and the loweringof large-current discharge properties, it is desired to add a smallquantity of the element A or B, or F, and to make them present only onthe surface. Among them, the elements A and B are preferably presentwithin 100 nm, more preferably within 30 nm from the particle surface.

A part of the elements A and B present on the particle surface of thepositive electrode active material is preferably a solid solution thatsubstitutes cobalt atoms in the particles. Also a part of fluorine atomspreferably substitutes for oxygen atoms in the particles to form a solidsolution.

In such a case, since no cobalt or oxygen atoms are exposed on theparticle surface of the positive electrode active material, the effectof the added elements is more preferably obtained. As a result, thecycle properties as the positive electrode active material for highvoltage uses are effectively improved. The addition of fluorine atoms ispreferable because it has the effect of improving the safety and thecycle properties of the cell.

It has been known that the atomic ratios of element-A atoms andelement-B atoms to cobalt atoms contained in the particulate positiveelectrode active material of the present invention (c/b and d/b) must be0.0001 to 0.02, respectively, these must be simultaneously added, andthe atomic ratio of the total quantity of element-A atoms and element-Batoms to cobalt atoms ((c+d)/b) must be 0.0001 to 0.02.

The element-A atomic ratio and the element-B atomic ratio smaller than0.0001, respectively, are not preferable, because the improving effectrelated to high cycle properties is reduced. On the other hand, theatomic ratio of the total quantity of element-A atoms and element-Batoms exceeding 0.02 is not preferable, because the initial capacity issignificantly lowered.

The atomic ratios of fluorine atoms and cobalt atoms are preferably0.0001 to 0.02, and more preferably 0.0005 to 0.008, for improvingsafety and cycle properties. The atomic ratio of fluorine exceeding thisvalue is not preferable, because the discharge capacity is significantlylowered.

Furthermore, it is preferable that the particulate positive electrodeactive material of the present invention has a press density of 2.7 to3.3 g/cm³. If the press density is lower than 2.7 g/cm³, it is notpreferable because the initial volume capacity density of the positiveelectrode when the positive electrode sheet is formed using theparticulate positive electrode active material is lowered, and on thecontrary, if the press density is higher than 3.3 g/cm³, it is notpreferable because the initial weight capacity density of the positiveelectrode is lowered, or the high-rate discharge property is lowered.Above all, the press density of the particulate positive electrodeactive material is preferably 2.9 to 3.2 g/cm³.

In the present invention, it is preferable to use substantiallyspherical cobalt oxyhydroxide wherein a large number of primaryparticles are coagulated to form a secondary particle as the cobaltmaterial, because the press density can be high. Here, the press densitymeans a numerical value obtained from the volume and powder weight whenthe powder is compressed under a pressure of 0.32 t/cm².

In addition, the specific surface area of the particulate positiveelectrode active material of the present invention is preferably 0.2 to1 m²/g. If the specific surface is smaller than 0.2 m²/g, the dischargecapacity per initial unit weight is lowered; on the contrary, if thespecific surface exceeds than 1 m²/g, the discharge capacity per initialunit volume is also lowered, and the excellent positive electrode activematerial of the object of the present invention cannot be obtained. Thespecific surface area is more preferably 0.3 to 0.7 m²/g.

The method for manufacturing the particulate positive electrode activematerial of the present invention is not specifically limited, butmethods well known to the art can be used for manufacturing. Forexample, as the cobalt material, cobalt hydroxide, tricobalt tetroxide,and cobalt oxyhydroxide; especially, cobalt oxyhydroxide and cobalthydroxide, which can exert high cell performance, are preferable. Alsoas the cobalt material, the cobalt material consisting of particleswherein 10 or more primary particles are coagulated to form a secondaryparticle, and containing at least either cobalt oxyhydroxide or cobalthydroxide is preferable, because high cell performance can be attained.

As sources for the elements A and B, oxide, hydroxides, chlorides,nitrates, organic acid salts, oxyhydroxides, and fluorides are used;especially, hydroxides and fluorides, which can exert high cellperformance, are preferable. As a source for lithium, lithium carbonateand lithium hydroxide are preferable. Also as a source for fluorine,lithium fluoride, aluminum fluoride, and magnesium fluoride arepreferable.

A mixture of these source materials, preferably a mixture of at leastone selected from an oxide containing the element A or B and a hydroxidecontaining the element A or B, lithium fluoride, cobalt hydroxide,cobalt oxyhydroxide or cobalt oxide, and lithium carbonate is fired inan oxygen-containing atmosphere at 600 to 1050° C., preferably at 850 to1000° C. for preferably 4 to 48 hours, more preferably for 8 to 20 hoursto convert to a composite oxide. If a fluoride containing the element Aor B is used in lieu of the compound containing the element A or B andlithium fluoride, good cell performance can be obtained.

As the oxygen-containing atmosphere, the use of an oxygen-containingatmosphere containing 10% by volume or more, especially 40% by volume ormore oxygen is preferable. Such a composite oxide can satisfy thepresent invention by varying the kind of each material, mixedcomposition and firing conditions. Also in the present invention,preliminary firing can be performed before the above-described firing.It is preferable that preliminary firing is performed in an oxidizingatmosphere, at preferably 450 to 550° C. for preferably 4 to 20 hours.

The manufacture of the positive electrode active material of the presentinvention is not necessarily limited to the above-described method, butit can be manufactured by synthesizing a positive electrode activematerial using, for example a metal fluoride, an oxide and/or ahydroxide as materials, and further performing surface treatment using afluorinating agent such as fluorine gas, NF₃ and HF.

In manufacturing a material of a positive electrode for a secondarylithium-ion cell, which is a particulate positive electrode activematerial for a secondary lithium cell represented by a general formula,Li_(a)Co_(b)A_(c)B_(d)O_(e)F_(f) (A is Al or Mg, B is a group-IVtransition element, 0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03,0.0001≦d≦0.03, 1.98≦e≦2.02, 0≦f≦0.02, and 0.0001≦c+d≦0.03); and thatelement A, element B and fluorine are evenly present in the vicinity ofthe particle surfaces, the present invention also provides a method formanufacturing a material of a positive electrode for a secondary lithiumcell that consists of particles wherein 10 or more primary particles arecoagulated to form a secondary particle, characterized in that a cobaltmaterial containing at least either cobalt oxyhydroxide or cobalthydroxide, lithium carbonate, and said element A and element B are mixedand fired.

The method for obtaining the positive electrode for a secondary lithiumcell from the particulate positive electrode active material of thepresent invention can be carried out according to the usual manner. Forexample, by mixing a carbon-based conductive material, such as acetyleneblack, graphite, and kitchen black, and a binder to the powder of thepositive electrode active material of the present invention, a positiveelectrode mixture is formed. As the binder, polyvinylidene fluoride,polytetrafluoroethylene, polyamide, carboxymethyl cellulose, acrylicresins or the like is used.

A slurry formed by dispersing the above-described positive electrodemixture in a dispersant such as N-methyl pyrrolidone is applied to apositive electrode collector, such as an aluminum foil, dried andpress-rolled to form a positive electrode active material layer on thepositive electrode collector.

In a lithium cell using the positive electrode active material of thepresent invention as the positive electrode, the solvent of theelectrolyte solution is preferably a carbonate ester. Both cyclic andchain carbonate esters can be used. As cyclic carbonate esters,propylene carbonate, ethylene carbonate (EC) and the like areexemplified. As chain carbonate esters, dimethyl carbonate, diethylcarbonate (DEC), ethyl methyl carbonate, methyl propyl carbonate, methylisopropyl carbonate and the like are exemplified.

The above-described carbonate esters can be used alone, or by mixing twoor more. They can also be used by mixing to other solvents. Depending onthe material of the negative electrode active material, the concomitantuse of a chain ester carbonate and a cyclic ester carbonate may improvethe discharge properties, cycle durability, charge-discharge efficiencyor the like.

A gel polymer electrolyte formed by adding a vinylidenefluoride-hexafluoropropylene copolymer (e.g., Kynar manufactured byAtochem), and a vinylidene fluoride-perfluoropropyl vinyl ethercopolymer to these organic solvents, and adding a solute described belowcan also be used.

As the solute of the electrolyte solution, it is preferable to useeither one or more lithium salt containing ClO₄—, CF₃SO₃—, BF₄—, PF₆—,AsF₆—, SbF₆—, CF₃CO₂—, (CF₃SO₂)₂N— or the like as anions. Theabove-described electrolyte solutions or polymer electrolytes arepreferably formed by adding an electrolyte consisting of a lithium saltto the above-described solvent or solvent-containing polymer at aconcentration of 0.2 to 2.0 mol/L. If the concentration is beyond thisrange, the ionic conductivity lowers, and the electric conductivity ofthe electrolyte lowers. More preferably, 0.5 to 1.5 mol/L is selected.For the separator, a porous polyethylene or porous polypropylene film isused.

The negative electrode active material of the lithium cell using thepositive electrode active material of the present invention as thepositive electrode is a material that can occlude and release lithiumions. Although the material forming the negative electrode activematerial is not specifically limited, for example, lithium metal,lithium alloys, carbon materials, oxides based on the metals in groups14 and 15 of the periodic table, carbon compounds, silicon carbidecompounds, silicon oxide compounds, titanium sulfide, boron carbidecompound and the like can be listed.

As carbon compounds, organic materials thermally decomposed undervarious conditions, artificial graphite, natural graphite, soilgraphite, expanded graphite, scale-like graphite or the like can beused. As oxides, compounds based on tin oxide can be used. As thenegative electrode collector, copper foil, nickel foil or the like canbe used.

The shape of the secondary lithium cell using the positive electrodeactive material of the present invention is not specifically limited. Asheet shape (i.e., film shape), folded shape, winding cylindrical shapewith a bottom, button shape or the like is selected depending on theuse.

EXAMPLES

Next, the specific examples 1 to 11, and the comparative examplesthereof 1 to 8 will be described below.

Example 1

Predetermined quantities of cobalt oxyhydroxide powder having an averageparticle diameter D50 of 10.2 μm, wherein 50 or more primary particleswere coagulated to form a secondary particle, lithium carbonate powder,aluminum hydroxide powder, and zirconium oxide power were mixed. Thesefour kinds of powders were dry-mixed, and then, fired at 950° C. for 14hours in the atmosphere. As a result of wet-dissolving the powder afterfiring, and measuring the content of cobalt, aluminum, zirconium andlithium by ICP and atomic absorption spectrometry, the composition ofthe powder was LiCo_(0.99)Al_(0.005)Zr_(0.005)O₂.

The specific surface area of the powder after firing (positive electrodeactive material powder) measured using the nitrogen adsorption method ofthe powder was 0.37 m²/g, and the average particle diameter D50 measuredusing a laser scattering type particle-size distribution analyzer was11.8 μm. As a result of XPS analysis of the powder surface after firing,a strong signal of Al2P caused by aluminum and a strong signal of Zr3Pcaused by zirconium were detected.

After the powder was sputtered for 10 minutes, XPS analysis wasconducted. The signals of aluminum and zirconium by XPS were attenuatedto 10% and 13% the signals before sputtering, respectively. Thesputtering is equivalent to surface etching to a depth of about 30 nm.It was known therefrom that aluminum and zirconium were present on thesurfaces of the particles. Furthermore, as a result of observation usingan SEM (scanning electron microscope), the obtained positive electrodeactive material formed secondary particles wherein 30 or more primaryparticles were coagulated.

The LiCo_(0.99)Al_(0.005)Zr_(0.005)O₂ powder thus obtained, acetyleneblack, and polytetrafluoroethylene powder were mixed in a weight ratioof 80/16/4, kneaded while adding toluene, and dried to fabricate apositive electrode plate of a thickness of 150 μm.

Then, using an aluminum foil of a thickness of 20 μm as a positiveelectrode collector, using porous polypropylene of a thickness of 25 μmas a separator, using a metallic lithium foil of a thickness of 500 μmas a negative electrode, using a nickel foil of 20 μm as a negativeelectrode collector, and using 1M LiPF₆/EC+DEC (1:1) as an electrolyte,a simple sealed cell made of stainless steel was assembled in an argonglove box.

The cell was first charged to 4.5 V using a load current of 75 mA for 1g of the positive electrode active material at 25° C., and discharged to2.75 V using a load current of 75 mA for 1 g of the positive electrodeactive material to obtain the initial discharge capacity. Furthermore,50 times of charge-discharge cycle tests were conducted.

The initial discharge capacity at 25° C. 2.75 to 4.5 V, and a dischargerate of 0.5 C was 186.4 mAh/g, and the average voltage was 4.019 V. Thecapacity retention after 50 times of charge-discharge cycles was 86.9%.

Another cell of the same type was fabricated. This cell was charged at4.3 V for 10 hours, and disassembled in an argon glove box. The positiveelectrode body sheet after charging was removed, and after washing thepositive electrode body sheet, it was punched into a diameter of 3 mm,sealed in an aluminum capsule together with EC, and the temperature wasraised at a rate of 5° C./min using a scanning differential calorimeterto measure the heat-generation starting temperature.

As a result thereof, the heat-generation starting temperature of the4.3-V charged material was 166° C.

Example 2

A positive electrode active material was synthesized in the same manneras in Example 1 except that magnesium hydroxide was used in lieu ofusing aluminum hydroxide, and composition analyses, propertymeasurements, and cell performance tests were carried out. As a resultthereof, the composition was LiCo_(0.99)Mg_(0.005)Zr_(0.005)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.32 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.5 μm. Magnesium and zirconiumwere present on the surface. The initial discharge capacity at 25° C.,2.75 to 4.5 V, and a discharge rate of 0.5 C was 192.0 mAh/g, and theaverage voltage was 4.009 V. The capacity retention after 50 times ofcharge-discharge cycles was 92.0%.

The X-ray diffraction spectrum of the fired powder was obtained by ahigh-sensitivity X-ray diffractometry using Cu—K α ray, using a ModelRINT2500 X-ray diffractometer manufactured by Rigaku Corporation underthe conditions of an accelerating voltage of 50 kV, an acceleratingcurrent of 250 mA, a scanning speed of 1°/min, a step angle of 0.02°, adivergence slit of 1°, a scattering slit of 1°, a receiving slit of 0.3mm, and monochromatization. The obtained spectrum is shown in FIG. 1.From FIG. 1, no diffraction spectrum at 2 θ of 28±1° was observed, andit was found that zirconium was not present as a single-component oxide.

It was also found from the analysis of the X-ray diffraction spectrumthat about 90% of the zirconium formed a solid solution with cobalt, andabout 10% thereof was present as Li₂ZrO₃. As a result of observationthrough SEM, 30 or more primary particles were coagulated to form asecondary particle in the obtained positive electrode active materialpowder.

Example 3

A positive electrode active material was synthesized in the same manneras in Example 1 except that predetermined quantities of cobaltoxyhydroxide powder having an average particle diameter D50 of 10.7 μmwherein 50 or more primary particles were coagulated to form a secondaryparticle, lithium carbonate powder, aluminum hydroxide powder, zirconiumoxide powder, and lithium fluoride powder were mixed; and compositionanalyses, property measurements, and cell performance tests were carriedout. As a result thereof, the composition wasLiCo_(0.99)Al_(0.005)Zr_(0.005)O_(1.9924)F_(0.0076).

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.34 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.8 μm. Aluminum, zirconium andfluorine were present on the surface. As a result of observation throughSEM, 30 or more primary particles were coagulated to form a secondaryparticle in the obtained positive electrode active material powder.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 185.6 mAh/g, and the average voltage was 4.02 V. Thecapacity retention after 50 times of charge-discharge cycles was 88.0%.The heat-generation starting temperature of the 4.3-V charged materialwas 173° C.

Example 4

A positive electrode active material was synthesized in the same manneras in Example 1 except that predetermined quantities of cobaltoxyhydroxide powder having an average particle diameter D50 of 10.7 μmwherein 50 or more primary particles were coagulated to form a secondaryparticle, lithium carbonate powder, magnesium hydroxide powder,zirconium oxide powder, and lithium fluoride powder were mixed; andcomposition analyses, property measurements, and cell performance testswere carried out. As a result thereof, the composition wasLiCo_(0.99)Mg_(0.005)Zr_(0.005)O_(1.9924)F_(0.0076).

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.35 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.7 μm. Magnesium and zirconiumwere present on the surface. As a result of observation through SEM, 30or more primary particles were coagulated to form a secondary particlein the obtained positive electrode active material powder.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 189.1 mAh/g, and the average voltage was 4.011 V. Thecapacity retention after 50 times of charge-discharge cycles was 91.6%.

Example 5

A positive electrode active material was synthesized in the same manneras in Example 1 except that hafnium oxide powders were used in lieu ofusing zirconium oxide, and composition analyses, property measurements,and cell performance tests were carried out. As a result thereof, thecomposition was LiCo_(0.99)Al_(0.005)Hf_(0.005)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.39 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.2 μm. Aluminum and hafniumwere present on the surface. As a result of observation through SEM, 30or more primary particles were coagulated to form a secondary particlein the obtained positive electrode active material powder.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 190.4 mAh/g, and the average voltage was 4.029 V. Thecapacity retention after 50 times of charge-discharge cycles was 88.0%.

Example 6

A positive electrode active material was synthesized in the same manneras in Example 5 except that magnesium hydroxide was used in lieu ofusing aluminum hydroxide, and composition analyses, propertymeasurements, and cell performance tests were carried out. As a resultthereof, the composition was LiCo_(0.99)Mg_(0.005)Hf_(0.005)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.41 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.4 μm. Magnesium and hafniumwere present on the surface. As a result of observation through SEM, 30or more primary particles were coagulated to form a secondary particlein the obtained positive electrode active material powder.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 189 mAh/g, and the average voltage was 4.009 V. Thecapacity retention after 50 times of charge-discharge cycles was 90.5%.

Example 7

A positive electrode active material was synthesized in the same manneras in Example 1 except that titanium oxide powders were used in lieu ofusing zirconium oxide, and composition analyses, property measurements,and cell performance tests were carried out. As a result thereof, thecomposition was LiCo_(0.99)Al_(0.005)Ti_(0.005)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.41 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.1 μm. Aluminum and titaniumwere present on the surface. As a result of observation through SEM, 30or more primary particles were coagulated to form a secondary particlein the obtained positive electrode active material powder.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 187.6 mAh/g, and the average voltage was 4.008 V. Thecapacity retention after 50 times of charge-discharge cycles was 88.2%.

Example 8

A positive electrode active material was synthesized in the same manneras in Example 7 except that magnesium hydroxide was used in lieu ofusing aluminum hydroxide, and composition analyses, propertymeasurements, and cell performance tests were carried out. As a resultthereof, the composition was LiCo_(0.99)Mg_(0.005)Ti_(0.005)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.43 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.0 μm. Magnesium and titaniumwere present on the surface. As a result of observation through SEM, 30or more primary particles were coagulated to form a secondary particlein the obtained positive electrode active material powder.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 187.3 mAh/g, and the average voltage was 4.005 V. Thecapacity retention after 50 times of charge-discharge cycles was 86.5%.

Example 9

A positive electrode active material was synthesized in the same manneras in Example 1 except that the quantities of aluminum hydroxide andzirconium oxide were changed, and composition analyses, propertymeasurements, and cell performance tests were carried out. As a resultthereof, the composition was LiCo_(0.98)Al_(0.01)Zr_(0.01)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.39 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.6 μm. Magnesium and titaniumwere present on the surface. As a result of observation through SEM, 30or more primary particles were coagulated to form a secondary particlein the obtained positive electrode active material powder.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 185.3 mAh/g, and the average voltage was 4.022 V. Thecapacity retention after 50 times of charge-discharge cycles was 86.5%.

Example 10

A positive electrode active material was synthesized in the same manneras in Example 2 except that the quantities of magnesium hydroxide andzirconium oxide were changed, and composition analyses, propertymeasurements, and cell performance tests were carried out. As a resultthereof, the composition was LiCo_(0.98)Mg_(0.01)Zr_(0.01)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.35 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.8 μm. Magnesium and zirconiumwere present on the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 186.6 mAh/g, and the average voltage was 4.003 V. Thecapacity retention after 50 times of charge-discharge cycles was 87.1%.

In the same manner as in Example 2, the X-ray diffraction spectrum ofthe powder after firing was obtained by a high-sensitivity X-raydiffractometry using Cu—K α ray. The obtained spectrum is shown in FIG.2. From FIG. 2, no diffraction spectrum at 2 θ of 28±1° was observed,and it was found that zirconium was not present as a single-componentoxide.

It was also found from the analysis of the X-ray diffraction spectrumthat about 90% of the zirconium formed a solid solution with cobalt, andabout 10% thereof was present as Li₂ZrO₃. As a result of observationthrough SEM, 30 or more primary particles were coagulated to form asecondary particle in the obtained positive electrode active materialpowder.

Example 11

A positive electrode active material was synthesized in the same manneras in Example 2 except that cobalt hydroxide having an average particlediameter D 50 of 12.7 μm, wherein 100 or more primary particles werecoagulated to form a secondary particle, was used as a cobalt source inlieu of cobalt oxyhydroxide, and composition analyses, propertymeasurements, and cell performance tests were carried out. As a resultthereof, the composition was LiCo_(0.99)Mg_(0.005)Zr_(0.005)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.43 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 14.8 μm. Magnesium and zirconiumwere present on the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 190.0 mAh/g, and the average voltage was 4.013 V. Thecapacity retention after 50 times of charge-discharge cycles was 93.3%.

In the same manner as in Example 2, the X-ray diffraction spectrum ofthe powder after firing was obtained by a high-sensitivity X-raydiffractometry using Cu—K α ray. As a result, no diffraction spectrum at2 θ of 28±1° was observed, and it was found that zirconium was notpresent as a single-component oxide.

As a result of observation through SEM, 30 or more primary particleswere coagulated to form a secondary particle in the obtained positiveelectrode active material powder. The packing properties of theelectrode layer using the positive electrode active material powderssynthesized in Example 11 were better than those of Example 2.

Comparative Example 1

A positive electrode active material was synthesized in the same manneras in Example 1 except that aluminum hydroxide and zirconium oxide werenot used; and composition analyses, property measurements, and cellperformance tests were carried out. As a result thereof, the compositionwas LiCoO₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.32 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 13.3 μm.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 194.5 mAh/g, and the average voltage was 4.008 V. Thecapacity retention after 50 times of charge-discharge cycles was 74.4%.The heat-generation starting temperature of the 4.3-V charged materialwas 163° C.

Comparative Example 2

A positive electrode active material was synthesized in the same manneras in Example 1 except that zirconium oxide was not used; andcomposition analyses, property measurements, and cell performance testswere carried out. As a result thereof, the composition wasLiCo_(0.99)Al_(0.01)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.32 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 13.4 μm. Aluminum was present onthe surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 189.0 mAh/g, and the average voltage was 4.016 V. Thecapacity retention after 50 times of charge-discharge cycles was 84.2%.

Comparative Example 3

A positive electrode active material was synthesized in the same manneras in Example 2 except that zirconium oxide was not used; andcomposition analyses, property measurements, and cell performance testswere carried out. As a result thereof, the composition wasLiCo_(0.99)Mg_(0.01)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.29 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 13.3 μm. Magnesium was presenton the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 190.1 mAh/g, and the average voltage was 3.980 V. Thecapacity retention after 50 times of charge-discharge cycles was 74.7%.

Comparative Example 4

A positive electrode active material was synthesized in the same manneras in Example 1 except that aluminum hydroxide was not used; andcomposition analyses, property measurements, and cell performance testswere carried out. As a result thereof, the composition wasLiCo_(0.99)Zr_(0.01)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.41 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 13.0 μm. Zirconium was presenton the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 186.4 mAh/g, and the average voltage was 4.022 V. Thecapacity retention after 50 times of charge-discharge cycles was 66.4%.

In the same manner as in Example 2, the X-ray diffraction spectrum ofthe powder after firing was obtained by a high-sensitivity X-raydiffractometry using Cu—K α ray. The obtained spectrum is shown in FIG.3. From FIG. 3, diffraction spectrum at 2 θ of 28±1° were significantlyobserved, and it was found that about 40% of zirconium was present as asingle-component oxide. It was also found from the analysis of the X-raydiffraction spectrum that about 50% of the zirconium formed a solidsolution with cobalt, and about 10% thereof was present as Li₂ZrO₃.

Comparative Example 5

A positive electrode active material was synthesized in the same manneras in Example 5 except that aluminum hydroxide was not used; andcomposition analyses, property measurements, and cell performance testswere carried out. As a result thereof, the composition wasLiCo_(0.99)Hf_(0.01)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.43 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 11.7 μm. Hafnium was present onthe surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 190.4 mAh/g, and the average voltage was 4.027 V. Thecapacity retention after 50 times of charge-discharge cycles was 82.7%.

Comparative Example 6

A positive electrode active material was synthesized in the same manneras in Example 8 except that magnesium hydroxide was not used; andcomposition analyses, property measurements, and cell performance testswere carried out. As a result thereof, the composition wasLiCo_(0.99)Ti_(0.01)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.50 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 12.7 μm. Titanium was present onthe surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 186.4 mAh/g, and the average voltage was 4.020 V. Thecapacity retention after 50 times of charge-discharge cycles was 78.3%.

Comparative Example 7

A positive electrode active material was synthesized in the same manneras in Example 3 except that aluminum hydroxide and zirconium oxide werenot used; and composition analyses, property measurements, and cellperformance tests were carried out. As a result thereof, the compositionwas LiCoO_(1.9924)F_(0.0076).

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.33 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 13.2 μm. Fluorine was present onthe surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 192.6 mAh/g, and the average voltage was 4.012 V. Thecapacity retention after 50 times of charge-discharge cycles was 78.2%.

Comparative Example 8

A positive electrode active material was synthesized in the same manneras in Example 1 except that the quantities of aluminum hydroxide andzirconium oxide were changed; and composition analyses, propertymeasurements, and cell performance tests were carried out. As a resultthereof, the composition was LiCo_(0.96)Al_(0.02)Zr_(0.02)O₂.

The specific surface area of the powder after firing measured using thenitrogen adsorption method of the powder was 0.44 m²/g, and the averageparticle diameter D50 measured using a laser scattering typeparticle-size distribution analyzer was 11.9 μm. Aluminum and zirconiumwere present on the surface.

The initial discharge capacity at 25° C., 2.75 to 4.5 V, and a dischargerate of 0.5 C was 181.9 mAh/g, and the average voltage was 4.021 V. Thecapacity retention after 50 times of charge-discharge cycles was 83.3%.

INDUSTRIAL APPLICABILITY

According to the present invention, as described above, there isprovided a material of a positive electrode for a secondary lithium-ioncell having high cycle durability and high safety for high-voltage andhigh-capacity uses, which are useful for secondary lithium-ion cells.

1. A material of a positive electrode for a secondary lithium cell,comprising a particulate active material of positive electrode for asecondary lithium-ion cell represented by a general formula,Li_(a)Co_(b)A_(c)B_(d)O_(e)F_(f), wherein A is Al or Mg, B is a group-IVtransition element, 0.90≦a≦1.10, 0.97≦b≦1.00, 0.0001≦c≦0.03,0.0001≦d≦0.03, 1.98≦e≦2.02, 0<f≦0.02, and 0.002≦c+d≦0.02, said elementA, element B and fluorine are evenly present in a vicinity of particlesurfaces; a single-component oxide of said element B is 20% or less; andno diffraction peaks are observed at 2θ of 28±1° in a high-sensitivityX-ray diffraction spectrum using Cu-Kα ray.
 2. The material of apositive electrode for a secondary lithium cell according to claim 1,wherein at least a part of said element represented by A or B containedin said particulate active material of the positive electrode for thesecondary lithium-ion cell has substituted for cobalt atoms in theparticles to form a solid solution.
 3. The material of a positiveelectrode for a secondary lithium cell according to claim 1, wherein theatomic ratio of said element A to said element B is 0.33≦c/d≦3.00,provided that 0.002≦c+d≦0.02.
 4. The material of a positive electrodefor a secondary lithium cell according to claim 1, wherein saidparticulate active material of the positive electrode for the secondarylithium-ion cell consists of secondary particles each formed bycoagulation of 10 or more primary particles, and an average particlediameter of said secondary particle is from 2 to 20 μm.